1. Field of the Invention
This invention relates to heat treating furnaces of the type in which a gas is circulated within a chamber, and more particularly to apparatus and process for batch annealing of metal products.
2. Description of the Prior Art
During various metalworking processes, weaknesses are induced in metals which could result in unanticipated and unnecessary failure of the metal. In order to eliminate these weaknesses, the metal is annealed by being heated and subsequently cooled in a controlled manner so that the effect of the heat on the metal eliminates the weaknesses. In order to improve the annealing efficiency, it is desirable to improve the heat transfer in order to reduce the time involved with the process.
It is common practice, for instance, when annealing rolls of sheet metal, to stack the rolls one on top of the other, edgewise inside a furnace. A heat transfer gas, which may be air or any inert gas, is circulated by fan to improve the heat transfer during heat-up and cool-down of the metal.
The density of the gas in the chamber is not constant at all times, but instead changes in accordance with factors such as temperature and pressure. Because the density of the gas varies, difficulties are introduced to the process. For instance, it is difficult to precisely match the design of the fan and the capacity of the fan motor to the load presented by the gas. In prior furnaces, the fan motor either possessed too much capacity and thus was expensive both in first cost and in operating costs, or the motor repeatedly approached an overload condition and experienced a short service life.
One method used is disclosed in U.S. Pat. No. 4,141,539. Means are provided to sense the magnitude of current drawn by the motor. The load on the motor is modified by changing the density of the gas, generally by changing the pressure within the chamber. This device certainly protected the motor but did not do much in the way of improving the efficiency of the overall batch-annealing process. Further, it was only applicable in those cases in which gas was continuously admitted to and exhausted from the chamber during the batch-annealing process. Accordingly, it required a vacuum source and a pressure source which added to the cost of the furnace.
A further difficulty presented by the changing density of the gas in the chamber relates to the heat transfer characteristics of a gas as its density changes. Generally speaking, the heat transfer capability of a gas decreases as its density decreases. Thus, as the temperature of the gas in the chamber increases, the gas density decreases and the heat transfer capability also decreases. For the state of the art heat treatment furnace, the fan speed remains constant, however, the heat transfer capacity of gas moved by the fan decreases because the density has decreased.
What is needed is a process and apparatus for improving the efficiency of the heat treating process. What is further needed is a device which will compensate efficiently for the change in the density of the heat transfer gas related to the gas temperature.